Optical fiber is generally preferred as the transmission medium for optical communication networks to satisfy the demand of substantial and fast-growing internet protocol (“IP”) data services. Wavelength division multiplexing (“WDM”), which combines many optical signals at different wavelength for transmission in a single optical fiber, is being used to meet the increasing demands for high speed and wide bandwidth in optical transport networks (“OTN”) applications. To fully exploit the increased capacity of OTN, a method of routing IP packets is needed so that optical signals arriving at the interconnected node can be transmitted. In order to carry out the plan of IP over WDM, different methods of optical switching take into consideration. The two main kinds of switching are classified: the optical circuit switching and the optical packet switching. Generally speaking, optical circuit switching means that the light from one wavelength or one optical fiber is coupled to another wavelength or another optical fiber. Optical packet switching means that the optical packet data from one time slot on one wavelength in one optical fiber can be transferred to another time slot on the same/or another wavelength in the same/or another optical fiber. Theoretically, the optical packet switching is much better than the optical circuit switching. Optical packet switching can allocate the huge bandwidth of dense wavelength division multiplexing (“DWDM”) more actively and more efficiently, but it raises a request for high quality optical devices especially for the optical switches.
Nowadays there is something new to be developed on optical switches. First of all, the dimension size of optical switching matrix should become large enough due to the use of high wavelength density of DWDM technologies and the advanced technology of wide-band optical amplifier; secondly, the ability to select the wavelength of optical switches is also necessary; thirdly, the demand for speed of the switch should be fast from the early level of millisecond (“ms”) even to the level of nanosecond (“ns”). There are several types of optical switches available commercially or under research as follows: Micro-Electrical-Mechanical-System (“MEMS”), “Inject BUBBLE”, optical liquid-crystal switch and thermal-optic optical switch etc. An approach of MEMS optical switch is to move the mini mirrors mechanically by electrical control. Another approach of Inject BUBBLE optical switch is to switch the light by moving the bubble into the slot via Total Internal Reflection (“TIR”). Optical liquid-crystal switch is to be realized by the use the optical polarization changing. Thermal-optic optical switch is due to the index change by thermal-optic effect. Pity that the majority kinds of optical switches mentioned above has slow switching time. There are further planar waveguide optical switches, e.g. Lithium Niobate (“LiNbO3”) or Indium Phosphorous (“InP”) planar based switch, which do have the fast switching ability less then 10 nanosecond or faster. However, these switches have no ability of wavelength selection according to publications. Recently, publications and patents about fast tunable wavelength selective optical switches around the world are very few. In the Journal of Lightwave Technology, (Vol. 14, no.6, pp1005,1996 ), David A. Smith, a researcher from Case Western Reserve University, presented a wavelength selective optical switch based on the effect of acoustic-optic. The acoustic-optic tuning scale-factor is 127 KHz/nm about a center frequency of 175 MHz in the 1550 nm-wavelength band. The switching speed of this switch is equal to the acoustic transit time (the given example shows 6 ms). There are three relating patents. The first one is U.S. Pat. No. 6,320,996, called “Wavelength Selective Optical Switch”. It is invented by Michael A Scobey of Optical Coating Laboratory in 2001. It chooses a certain wavelength light while reflects all the others by using a mechanical moving thin-film narrow-band wavelength filter. The major disadvantage of this switch is that it takes too long time for the switch to response due to the use of mechanical moving parts inside; also the wavelength of light, which has been chosen, is always fixed. The second one is U.S. Pat. No. 6,327,019, called “Dual liquid-crystal wavelength selective optical switch”. It is invented by Jayantilal Patel, W. John Tomlinson, Janet Lehr Jackel, etc., from Tellium Inc and Telcordia Technologies in 2001. It is made up of frequency-dispersive grating, Wollaston prism (used as polarization dispersive element) and liquid-crystal polarized modulation. Since it uses liquid crystal, the stability and responding speed are relatively lower; also the selected-wavelength of the switch is fixed, not tunable. The third patent is WO 0192951, called “Fabry-Perot optical switch”. It was invented by Iaconis Christopher, Swaby Basill, Adair Robert W, etc. of Optical Coating Laboratory Inc.(USA) in 2001. It applies a kind of absorbing materials, such as semiconductor, to be inserted into a Fabry-Perot cavity, which was electrically controlled and leads to the two states of transmission and reflection for a particular wavelength. However, the wavelength is also fixed once the cavity is set-up. So, such kind of optical switch has no tuning ability for the selected wavelength.
Thus, it is desirable to provide a kind of optical switch, which has the wavelength selection and further features. For example, the selected wavelength could be tuned fast up to nanosecond level to satisfy the further optical packet switching networks. In a preferred embodiment, the optical switch should have very low loss (e.g. less than 0.5 dB), fast switch speed (e.g. less than 10 ns), and wide wavelength tuning ability (e.g. 10 nanometer or wider).